Re: [f2fs-dev] [PATCH v2] f2fs: readahead node blocks in F2FS_GET_BLOCK_PRECACHE mode

[Chao Yu via Linux-f2fs-devel <linux-f2fs-devel@lists.sourceforge.net>]

On 9/24/25 12:17, Yunji Kang wrote:
>>> In f2fs_precache_extents(), For large files, It requires reading many
>>> node blocks. Instead of reading each node block with synchronous I/O,
>>> this patch applies readahead so that node blocks can be fetched in
>>> advance.
>>>
>>> It reduces the overhead of repeated sync reads and improves efficiency
>>> when precaching extents of large files.
>>>
>>> I created a file with the same largest extent and executed the test.
>>> For this experiment, I set the file's largest extent with an offset of
>>> 0 and a size of 1GB. I configured the remaining area with 100MB extents.
>>>
>>> 5GB test file:
>>> dd if=/dev/urandom of=test1 bs=1m count=5120 cp test1 test2 fsync
>>> test1 dd if=test1 of=test2 bs=1m skip=1024 seek=1024 count=100
>>> conv=notrunc dd if=test1 of=test2 bs=1m skip=1224 seek=1224 count=100
>>> conv=notrunc ...
>>> dd if=test1 of=test2 bs=1m skip=5024 seek=5024 count=100 conv=notrunc
>>> reboot
>>>
>>> I also created 10GB and 20GB files with large extents using the same
>>> method.
>>>
>>> ioctl(F2FS_IOC_PRECACHE_EXTENTS) test results are as follows:
>>>   +-----------+---------+---------+-----------+
>>>   | File size | Before  | After   | Reduction |
>>>   +-----------+---------+---------+-----------+
>>>   | 5GB       | 101.8ms | 72.1ms  | 29.2%     |
>>>   | 10GB      | 222.9ms | 149.5ms | 32.9%     |
>>>   | 20GB      | 446.2ms | 276.3ms | 38.1%     |
>>>   +-----------+---------+---------+-----------+
>>
>> Yunji,
>>
>> Will we gain better performance if we readahead more node pages w/
>> sychronous request for precache extent case? Have you tried that?
>>
>> Thanks,
>>
> 
> Does “readahead more node pages” mean removing this condition?
> " offset[i - 1] % MAX_RA_NODE == 0 "

Actually, I meant a) remove "offset[i - 1] % MAX_RA_NODE == 0" or b)
increase MAX_RA_NODE.

Also, maybe we can try as below to trigger synchronous IO for such high
determinacy read.

void df2fs_ra_node_page()
{
...
	err = read_node_folio(afolio, 0);
...
}

> 
> I originally added the condition to prevent unnecessary readahead requests, 
> but it seems this condition was actually blocking valid readahead as well.
> 
> After removing the condition and running tests, 
> I confirmed that more readahead node pages are being issued.
> 
> I’ll share the test results along with the improved patch.

It makes sense, thanks for checking this and sharing the result.

Thanks,

> 
> Thanks,
> 
>>> Tested on a 256GB mobile device with an SM8750 chipset.
>>>
>>> Reviewed-by: Sungjong Seo <sj1557.seo@samsung.com>
>>> Reviewed-by: Sunmin Jeong <s_min.jeong@samsung.com>
>>> Signed-off-by: Yunji Kang <yunji0.kang@samsung.com>
>>> ---
>>> v2:
>>>  - Modify the readahead condition check routine for better code
>>> readability.
>>>  - Update the title from 'node block' to 'node blocks'.
>>>
>>>  fs/f2fs/data.c | 3 +++
>>>  fs/f2fs/f2fs.h | 1 +
>>>  fs/f2fs/node.c | 5 ++++-
>>>  3 files changed, 8 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c index
>>> 7961e0ddfca3..ab3117e3b24a 100644
>>> --- a/fs/f2fs/data.c
>>> +++ b/fs/f2fs/data.c
>>> @@ -1572,6 +1572,9 @@ int f2fs_map_blocks(struct inode *inode, struct
>> f2fs_map_blocks *map, int flag)
>>>  	pgofs =	(pgoff_t)map->m_lblk;
>>>  	end = pgofs + maxblocks;
>>>
>>> +	if (flag == F2FS_GET_BLOCK_PRECACHE)
>>> +		mode = LOOKUP_NODE_PRECACHE;
>>> +
>>>  next_dnode:
>>>  	if (map->m_may_create) {
>>>  		if (f2fs_lfs_mode(sbi))
>>> diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h index
>>> 9d3bc9633c1d..3ce41528d48e 100644
>>> --- a/fs/f2fs/f2fs.h
>>> +++ b/fs/f2fs/f2fs.h
>>> @@ -651,6 +651,7 @@ enum {
>>>  					 * look up a node with readahead called
>>>  					 * by get_data_block.
>>>  					 */
>>> +	LOOKUP_NODE_PRECACHE,		/* look up a node for
>> F2FS_GET_BLOCK_PRECACHE */
>>>  };
>>>
>>>  #define DEFAULT_RETRY_IO_COUNT	8	/* maximum retry read IO or flush
>> count */
>>> diff --git a/fs/f2fs/node.c b/fs/f2fs/node.c index
>>> 4254db453b2d..d4bf3ce715c5 100644
>>> --- a/fs/f2fs/node.c
>>> +++ b/fs/f2fs/node.c
>>> @@ -860,7 +860,10 @@ int f2fs_get_dnode_of_data(struct dnode_of_data *dn,
>> pgoff_t index, int mode)
>>>  			set_nid(parent, offset[i - 1], nids[i], i == 1);
>>>  			f2fs_alloc_nid_done(sbi, nids[i]);
>>>  			done = true;
>>> -		} else if (mode == LOOKUP_NODE_RA && i == level && level > 1)
>> {
>>> +		} else if ((i == level && level > 1) &&
>>> +				(mode == LOOKUP_NODE_RA ||
>>> +				(mode == LOOKUP_NODE_PRECACHE &&
>>> +				offset[i - 1] % MAX_RA_NODE == 0))) {
>>>  			nfolio[i] = f2fs_get_node_folio_ra(parent, offset[i -
>> 1]);
>>>  			if (IS_ERR(nfolio[i])) {
>>>  				err = PTR_ERR(nfolio[i]);
> 
> 
> 



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